1. Field of the Invention
The invention relates to a two stage naphtha desulfurization process with reduced mercaptan formation. More particularly, the invention relates to a two stage, vapor phase naphtha hydrodesulfurization process with interstage H.sub.2 S removal, wherein the second stage temperature and space velocity is greater than the first, for deep, selective sulfur removal with reduced olefin loss and mercaptan formation.
2. Background of the Invention
Future mogas sulfur specifications are being regulated through legislation to increasingly lower levels, due to environmental considerations. Sulfur specifications on the order of less than 150 wppm of total sulfur are likely near term, with values of no greater than 30 wppm of total sulfur possible in the not too distant future. Such sulfur specifications are without precedent and will require the production of low sulfur blend stock for the mogas pool. The primary sulfur sources in the mogas pool are the blend stocks derived from FCC naphthas, whose sulfur content can fall in the range of 100-7000 wppm depending upon crude quality and FCC operation. Conventional fixed bed hydrodesulfurization can reduce the sulfur level of FCC naphthas to very low levels, but the severe conditions of temperature, pressure and treat gas ratio results in significant octane loss, due to extensive loss of olefins by saturation. Selective one and two-stage hydrodesulfurization processes have been developed to avoid massive olefin saturation and octane loss. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,149,965; 4,243,519; 5,525,211; 5,423,975, and 5,906,730. However, some of these processes, including two-stage processes, aren't suitable for use with high (e.g., &gt;1000 wppm) levels of feed sulfur. Further, in these and in other processes, in the hydrodesulfurization reactor the liberated H.sub.2 S reacts with the retained olefins, to form mercaptan sulfur compounds. These mercaptans are formed as a consequence of the H.sub.2 S formed during the hydrodesulfurization process and are known as reversion mercaptans. The amount of these mercaptans formed during the process typically exceeds future fuel specifications for mercaptan sulfur and, in some cases, total sulfur. Accordingly, a selective naphtha desulfurization process is needed for reducing the total sulfur level, particularly for high sulfur content naphtha feeds, with minimal mercaptan reversion, while retaining a high level of feed olefins.